Sigmoid alert and eruptive flares

Science Nugget: May 11, 2001

Our "campaign" strategy - observing TOO

Last week we finally got untracked and executed our carefully-laid
plans for a "Target Of Opportunity" (TOO) observing campaign, based
upon the recognition of a new "sigmoid" coronal structure in the
soft X-ray images.
This required sending a message through
Max Millennium, our information clearing-house for activity
during the current solar sunspot maximum.
The
message announcing the discovery of the sigmoid went out on May 3.
It incidentally coincided with a visit to ISAS by Dick Canfield, a guru
of
helicity as applied to the Sun, as is Alex Pevtsov, the author of
the page just linked.
Previous work has shown that the presence of bright S-shapes in X-ray
images of the Sun anticipates eruptions.
So our TOO has the purpose of orchestrating the response of many solar
observers to catch the origins of an eruptive flare and CME.
We were just lucky that Dick Canfield visited, shaping us up for this
sometime ambiguous observational chore, and also shaping up the Sun so
that it would cooperate.
We chose NOAA active region 9445, as seen in this not-so-sigmoidal image
from near its W limb passage:

The circle around the active region in the NE (upper right) shows where
AR 9445 was on May 8, the day of a C9.9 flare - the day after the alert
there had been an M-class eruptive flare, and two days later quite a
striking CME occurred.
All of this gladdens our hearts, we believers in the powers of a sigmoid!

What NOAA 9445 did

Some of these flares were not obviously CMEs in the SXT images.
Our criteria for this would be the formation of a cusp or an arcade,
or ideally the direct observation of things flying away from the site
of the flare.
An example is the M-class flare of May 8 (see the
standard plot); below is a
single frame from the SXT high-temperature Be filter:

Also there's a movie of this event:

shows clearly that this event is eruptive, but that the eruption
is a confined eruption.
In other words, the restructuring increases the size of the bright
loops, but it only goes so far and does not "blow out."
Interestingly, a CME did not appear to be associated directly with
this flare, although region 9445 seemed to be pumping CMEs out with
great abandon, almost as in a continual flow.
If you have good bandwidth, please check out the
beautiful SOHO/LASCO movie,
noting especially the amazing N pole streamer blowout about 12 hours
prior to this flare.

How does this help theorists understand eruptions?

An eruption can be confined?
This fuzzes out the distinction we thought we had clearly recognized
between flares and CMEs on the one hand, and energy and helicity on
the other.
This maybe mythical pattern could be written in shorthand as

In terms of theories, the morphology we see here has two contenders
at least.
Neither of them, of course, can determine quantitatively why any particular
eruption would be confined or non-confined.
One is the "tether-cutting" kind of reconnection, which involves only
a single magnetic domain that is intrinsically unstable (e.g. Moore);
another is the "separatrix" school of thought (e.g. Demoulin), in which
multiple domains contend.
Of course we hope that observations of the type recorded here will help
to show both ideas wrong!

Conclusions

Our alert system for sigmoids worked this time,
complementing our less-organized earlier efforts;
please see the search facilities of these nuggets to learn more about
our first go-around.
Given that what we see here differs somewhat from what we saw before,
we will have to conclude that one cannot have TOO many observations
of sigmoids!